Fibrinolytic therapy has found wide application in the treatment of both arterial and venous thrombotic diseases, resulting in significant improvement in mortality and morbidity. In preliminary experiments we have observed that ultrasound (US) at frequencies from l to 3.4 MHz accelerates enzymatic fibrinolysis in vitro and in vivo at intensities of 1 W/cm2 and above, primarily through non-thermal mechanisms. The goals of the proposed studies follow from these findings, and they are to determine the mechanism of the US effect and to define conditions for optimal application of US for eventual clinical use. The first specific aim is to determine the mechanism by which US accelerates fibrinolysis. We hypothesize that US increases transport of plasminogen and of plasminogen activator into the clot and also alters their distribution within the lot. This will be tested by examining the effects of US on the uptake of radiolabeled plasminogen and plasminogen activator into clots, on their binding to fibrin and on their spatial distribution. We also hypothesize that US affects the structure of fibrin fibers, and this will be examined using electron microscopy. Experiments will be conducted to determine the physical mechanism(s) by which US exerts these effects, examining possible contributions from heating, increasing chemical gradients, phonophoresis, microstreaming, inertial cavitation and enhanced flow through clots. The second specific aim is to characterize the parameters that influence ultrasonic enhancement of fibrinolysis in animal models. The effects of US on fibrinolysis will be tested in vivo using rabbit models of arterial and venous thrombosis and of hemostatic plug dissolution. The degree of ultrasonic acceleration of fibrinolysis in vivo will be characterized at several intensities and duty cycles, and we will also determine whether reduced activator doses are effective in the presence of US. Toxicity will be evaluated by assessment of tissue heating, systemic fibrinogenolysis and morphologic observation. The eventual non-invasive application of US to potentiate fibrinolysis has the potential to improve thrombolytic therapy by accelerating clot dissolution, increasing lysis of resistant clots and by decreasing activator dose.